Flash memory is a type of non-volatile memory that is electrically erasable and re-programmable. Flash memory is a specific type of electrically erasable programmable read-only memory (EEPROM) that is programmed and erased in large blocks. One commonly employed type of flash memory technology is NAND flash memory. NAND flash memory exhibits fast erase and write times, requires small chip area per cell, and has high endurance. Due to a greater storage capacity and lower cost than similar technologies, NAND flash memory forms the core of the flash memory available today, especially for removable universal serial bus (USB) storage devices and memory cards.
Flash memory stores information in an array of memory cells made from floating gate transistors. These transistors can hold their voltage level, also referred to as charge, for long periods of time, on the order of years, without external power being supplied. In single-level cell (SLC) flash memory, each cell stores one bit of information. In multi-level cell (MLC) flash memory, each cell can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells. MLC NAND flash memory employs multiple voltage levels per cell with a serially linked transistor arrangement to allow more bits to be stored using the same number of transistors. Most MLC NAND flash memory employs four possible states per cell, and can thus store two bits of information per cell. This level increase per cell causes cell-to-cell interference and retention noise to become more severe, which reduces the amount of voltage margin separating each voltage level and might increase the bit error ratio (BER). Software complexity might be increased to compensate for a larger BER, for example by employing an error correction code (ECC) such as Bose-Chaudhuri-Hocquenghem (BCH), Reed-Solomon (RS), and Low-Density Parity-Check (LDPC). Thus, there is a need to achieve lower BERs and increased performance of MLC NAND flash memory